Rearrange array such that even index elements are smaller and odd index elements are greater
Given an array, rearrange the array such that :
- If index i is even, arr[i] <= arr[i+1]
- If index i is odd, arr[i] >= arr[i+1]
Note: There can be multiple answers.
Examples:
Input : arr[] = {2, 3, 4, 5} Output : arr[] = {2, 4, 3, 5} Explanation : Elements at even indexes are smaller and elements at odd indexes are greater than their next elements Note : Another valid answer is arr[] = {3, 4, 2, 5} Input :arr[] = {6, 4, 2, 1, 8, 3} Output :arr[] = {4, 6, 1, 8, 2, 3}
This problem is similar to sorting an array in the waveform.
- A simple solution is to sort the array in decreasing order, then starting from the second element, swap the adjacent elements.
- An efficient solution is to iterate over the array and swap the elements as per the given condition.
If we have an array of length n, then we iterate from index 0 to n-2 and check the given condition.
At any point of time if i is even and arr[i] > arr[i+1], then we swap arr[i] and arr[i+1]. Similarly, if i is odd and
arr[i] < arr[i+1], then we swap arr[i] and arr[i+1].
For the given example:
Before rearrange, arr[] = {2, 3, 4, 5} Start iterating over the array till index 2 (as n = 4)
First Step:
At i = 0, arr[i] = 2 and arr[i+1] = 3. As i is even and arr[i] < arr[i+1], don’t need to swap.
Second step:
At i = 1, arr[i] = 3 and arr[i+1] = 4. As i is odd and arr[i] < arr[i+1], swap them.
Now arr[] = {2, 4, 3, 5}
Third step:
At i = 2, arr[i] = 3 and arr[i+1] = 5. So, don’t need to swap them
After rearrange, arr[] = {2, 4, 3, 5}
Flowchart

Flowchart
Implementation:
C++
// CPP code to rearrange an array such that // even index elements are smaller and odd // index elements are greater than their // next. #include <iostream> using namespace std; void rearrange( int * arr, int n) { for ( int i = 0; i < n - 1; i++) { if (i % 2 == 0 && arr[i] > arr[i + 1]) swap(arr[i], arr[i + 1]); if (i % 2 != 0 && arr[i] < arr[i + 1]) swap(arr[i], arr[i + 1]); } } /* Utility that prints out an array in a line */ void printArray( int arr[], int size) { for ( int i = 0; i < size; i++) cout << arr[i] << " " ; cout << endl; } /* Driver function to test above functions */ int main() { int arr[] = { 6, 4, 2, 1, 8, 3 }; int n = sizeof (arr) / sizeof (arr[0]); cout << "Before rearranging: \n" ; printArray(arr, n); rearrange(arr, n); cout << "After rearranging: \n" ; printArray(arr, n); return 0; } |
Java
// Java code to rearrange an array such // that even index elements are smaller // and odd index elements are greater // than their next. class GFG { static void rearrange( int arr[], int n) { int temp; for ( int i = 0 ; i < n - 1 ; i++) { if (i % 2 == 0 && arr[i] > arr[i + 1 ]) { temp = arr[i]; arr[i] = arr[i + 1 ]; arr[i + 1 ] = temp; } if (i % 2 != 0 && arr[i] < arr[i + 1 ]) { temp = arr[i]; arr[i] = arr[i + 1 ]; arr[i + 1 ] = temp; } } } /* Utility that prints out an array in a line */ static void printArray( int arr[], int size) { for ( int i = 0 ; i < size; i++) System.out.print(arr[i] + " " ); System.out.println(); } // Driver code public static void main(String[] args) { int arr[] = { 6 , 4 , 2 , 1 , 8 , 3 }; int n = arr.length; System.out.print( "Before rearranging: \n" ); printArray(arr, n); rearrange(arr, n); System.out.print( "After rearranging: \n" ); printArray(arr, n); } } // This code is contributed by Anant Agarwal. |
Python3
# Python code to rearrange # an array such that # even index elements # are smaller and odd # index elements are # greater than their # next. def rearrange(arr, n): for i in range (n - 1 ): if (i % 2 = = 0 and arr[i] > arr[i + 1 ]): temp = arr[i] arr[i] = arr[i + 1 ] arr[i + 1 ] = temp if (i % 2 ! = 0 and arr[i] < arr[i + 1 ]): temp = arr[i] arr[i] = arr[i + 1 ] arr[i + 1 ] = temp # Utility that prints out an array in # a line def printArray(arr, size): for i in range (size): print (arr[i], " " , end = "") print () # Driver code arr = [ 6 , 4 , 2 , 1 , 8 , 3 ] n = len (arr) print ( "Before rearranging: " ) printArray(arr, n) rearrange(arr, n) print ( "After rearranging:" ) printArray(arr, n); # This code is contributed # by Anant Agarwal. |
C#
// C# code to rearrange an array such // that even index elements are smaller // and odd index elements are greater // than their next. using System; class GFG { static void rearrange( int [] arr, int n) { int temp; for ( int i = 0; i < n - 1; i++) { if (i % 2 == 0 && arr[i] > arr[i + 1]) { temp = arr[i]; arr[i] = arr[i + 1]; arr[i + 1] = temp; } if (i % 2 != 0 && arr[i] < arr[i + 1]) { temp = arr[i]; arr[i] = arr[i + 1]; arr[i + 1] = temp; } } } /* Utility that prints out an array in a line */ static void printArray( int [] arr, int size) { for ( int i = 0; i < size; i++) Console.Write(arr[i] + " " ); Console.WriteLine(); } // Driver code public static void Main() { int [] arr = { 6, 4, 2, 1, 8, 3 }; int n = arr.Length; Console.WriteLine( "Before rearranging: " ); printArray(arr, n); rearrange(arr, n); Console.WriteLine( "After rearranging: " ); printArray(arr, n); } } // This code is contributed by vt_m. |
PHP
<?php // PHP code to rearrange an array such // that even index elements are smaller // and odd index elements are greater // than their next. function swap(& $a , & $b ) { $temp = $a ; $a = $b ; $b = $temp ; } function rearrange(& $arr , $n ) { for ( $i = 0; $i < $n - 1; $i ++) { if ( $i % 2 == 0 && $arr [ $i ] > $arr [ $i + 1]) swap( $arr [ $i ], $arr [ $i + 1]); if ( $i % 2 != 0 && $arr [ $i ] < $arr [ $i + 1]) swap( $arr [ $i ], $arr [ $i + 1]); } } /* Utility that prints out an array in a line */ function printArray(& $arr , $size ) { for ( $i = 0; $i < $size ; $i ++) echo $arr [ $i ] . " " ; echo "\n" ; } // Driver Code $arr = array (6, 4, 2, 1, 8, 3 ); $n = sizeof( $arr ); echo "Before rearranging: \n" ; printArray( $arr , $n ); rearrange( $arr , $n ); echo "After rearranging: \n" ; printArray( $arr , $n ); // This code is contributed // by ChitraNayal ?> |
Javascript
<script> // JavaScript code to rearrange an array such that // even index elements are smaller and odd // index elements are greater than their // next. let rearrange = (arr, n)=>{ for (let i = 0; i < n - 1; i++) { if (i % 2 == 0 && arr[i] > arr[i + 1]){ // swap(arr[i], arr[i + 1]); let temp = arr[i]; arr[i] = arr[i+1]; arr[i+1] = temp; } if (i % 2 != 0 && arr[i] < arr[i + 1]){ // swap(arr[i], arr[i + 1]); let temp = arr[i]; arr[i] = arr[i+1]; arr[i+1] = temp; } } } // function to print array let printArray = (arr, size)=>{ ans = '' ; for (let i = 0; i < size; i++){ ans += arr[i] + " " ; } document.write(ans); } // Driver Code let arr = [ 6, 4, 2, 1, 8, 3 ]; let n = arr.length; document.write( "Before rearranging: " ); printArray(arr, n); rearrange(arr, n); document.write( "<br>" ); document.write( "After rearranging: " ); printArray(arr, n); </script> |
Before rearranging: 6 4 2 1 8 3 After rearranging: 4 6 1 8 2 3
Time Complexity: O(N), as we are using a loop to traverse N times,
Auxiliary Space: O(1), as we are not using any extra space.
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